Marine electromagnetic exploration is an important tool for locating off-shore hydrocarbon reserves and monitoring hydrocarbon production for reservoir management. One known procedure for marine electromagnetic involves the use of an electromagnetic source and receiver cables as described in the present applicants' WO 01/57555. Electromagnetic energy generated by the source propagates both upwards into the water column and downwards through the earth. The downward propagating waves are partially reflected and refracted by subsurface layers. The reflected and refracted energy travels upwardly from the subsurface layers and is detected by the receiver array. In particular, hydrocarbon filled reservoirs are known to give strongly refracted energy which is of interest for hydrocarbon imaging.
Electromagnetic exploration however is complicated by waves received at the receiver array as downward-travelling reflections and refractions after reflecting and refracting off the air/water boundary at the surface. The air/water boundary is an efficient reflector and refractor, and thus the waves travelling downwards are difficult to differentiate from the upgoing waves from the subsurface. The downward-travelling energy is caused both by energy propagating directly from the electromagnetic source to the air/water boundary and by energy from the subsurface travelling to the air/water boundary.
Reflections and refractions from the sea surface thus are a severe problem. If the sea surface reflections and refractions are not properly attenuated, they may interfere and overlap with primary reflections and refractions from the subsurface.
It is an object of the present invention to provide a method of processing an EM wavefield which minimises this difficulty.